1. Field of the Invention
The invention relates to a low-field nuclear magnetic resonance system, in particular to a low-field nuclear magnetic resonance system for pre-magnetizing an object to be measured and capable of adjusting a resonance frequency and background magnetic field intensity during regulation nuclear magnetic resonance measurement.
2. Description of Related Art
From the viewpoint of quantum mechanics for the existing nuclear magnetic resonance (NMR) technique, as the proton number and the mass number are of an odd number, the magnetic quantum number (I) of a nucleus is not zero. For example, for hydrogen, I=½. In such a situation, if the nucleus is placed in an externally applied magnetic field, the spin magnetic torque will be split into two energy levels, i.e. a low energy level of forward magnetic field (upward spin magnetic torque) and a high energy level of reverse magnetic field (downward spin magnetic torque). The energy difference ΔE=hγB0, in which h is a reduced Planck Constant, and γ is a gyromagnetic ratio, for example, γ of 1H being equal to 42.58 MHz/Tesla. According to Boltzmann Distribution Law, the distributed population of the magnetic torque in the low energy is higher than that in the high energy and the ratio of N↓/N↑ is:
                    N        ↓                    N        ↑              =          exp      ⁡              (                  -                                    Δ              ⁢                                                          ⁢              E                        kT                          )              ,
wherein, ΔE is the energy difference between the upward spin magnetic torque and the downward spin magnetic torque, k is Boltzmann's Constant, 1.3805×10−23 J/Kelvin, and T is the temperature of a spin system (Kelvin's temperature standard).
Much of the distributed population is in the state of upward spin magnetic torque, resulting in net magnetization being in the z direction (as B0 is the z direction). At this time, if a pulse (B1) of the same precession frequency (Larmor frequency) of the nucleus is applied in the y direction, such that the nucleus obtains energy, the net magnetization will be deviated to the x axis and the angle of deviation is correlated with the time of pulse on and the strength thereof. As the angle of deviation of the net magnetization is 90°, it is called a 90° pulse. As the angle of deviation of the net magnetization is 180°, it is called a 180° pulse. The relation between the angle of deviation and the time t of the externally applied pulse is θ=γB1t. At pulse off, the net magnetization is effected by B0, rendering the magnetization of deviation to make precession along the direction of magnetization (z axis). Due to the precession of the magnetization incorporating with relaxation of the magnetization to be recovered to a balance state (z axis), the magnetization forms a track of a top. As the nucleus returns to a base state from the stimulated state, there are mainly two independent components surrounding the externally applied magnetic field for precession, i.e. spin-lattice relation, being a recovery of the z component with its relaxation time constant called longitudinal relation time T1, and spin-spin relaxation, in which the x-y component returns to zero with its relaxation time constant called transverse relaxation time T2. The relation of the net magnetization is:Mx′y′(t)=Mx′y′(O+)exp(−t/T2)Mz′(t)=Mz,o[1−exp(−t/T1)]+Mz′(O+)exp(−t/T2)
The track of the magnetization will direct an oscillation current to be recorded. Such situation is just like a projection on two mutually vertical planes of the top-like track. It is called a free induced declining (FID) signal. After Fourier Transformation of the FID signal, a spectrum signal of the NMR is obtained, as shown in FIG. 1.
Low-field NMR is a branch of the NMR and is regarding earth's field NMR or NMR in an extremely low, human-made magnetic field and being shielded (or compensated) by the earth field. Differing from the magnitude of the magnetic field traditionally used in NMR being in 1.5 T˜4 T, the low-field NMR makes use the magnitude of the magnetic field being in the grade of μT or nT, while making use of super conducting quantum interference devices as sensors. Meanwhile, the low-field NMR provides the following advantages: (1) narrow bandwidth of the magnetic resonance; (2) magnetizing artifact resulting from high magnetic field being tremendously reduced; and (3) no need of super conductive coil, i.e. reducing cost. However, the NMR still has a defect, i.e. the signal of the low-field NMR being far smaller than that of the traditional NMR system.